1. Field of the Invention
The present invention relates to an analog semiconductor integrated circuit device having a sensor or a power management function, and to a method of manufacturing the semiconductor integrated circuit device.
2. Description of the Related Art
When an analog semiconductor integrated circuit device having a sensor or a power management function is composed of MOS transistors, a so called multi-Vth method, in which a plurality of threshold voltages (hereinafter, referred to as Vth) of the MOS transistor, instead of one, are used, is generally applied in order to carry out complicated analog signal processing and to cover various input voltage ranges.
In a conventional semiconductor integrated circuit device as shown in FIGS. 7A and 7B, Vth value of a MOS transistor is determined by ion implantations 203 and 205. A plurality of photolithography steps and a plurality of ion implanting steps are performed to form a multi-Vth device (For example, refer to JP 2000-323587 A (p. 6, FIG. 2)).
In the prior art, a plurality of photolithography steps and a plurality of ion implanting steps to attain a multi-Vth device have increased the cost for manufacturing and have elongated a manufacturing period, which turns to be a problem in product delivery. Further, since the number of steps cannot be increased unnecessarily, it is essentially impossible to attain multi-Vth having too many threshold voltages.